Conventionally, getting sufficient power from an external cavity tunable laser may be difficult in a hybrid laser system, when the laser includes a bump bonding or edge coupling between a group III-V based reflective semiconductor optical amplifier (RSOA) and a laser cavity formed in a device layer of a photonic integrated circuit (PIC), e.g. silicon photonics (SiPh). The difficulty arises because at a given injection current the power is limited by: 1) coupling losses from the RSOA to the external cavity, 2) losses on the PIC, and 3) limited gain of the RSOA.
The first issue is a result of the hybrid integration of the laser onto the PIC, whereby the losses can be too high. The second issue is due to the optical power being too high in some areas of the laser, and hence excess losses are created that quickly limit optical power output and the wall plug efficiency of the laser. For example, higher power of the RSOA, and eventually higher power in PIC, can cause nonlinear losses in the PIC. Thirdly, it is difficult to achieve high optical gain from RSOAs in a silicon photonics (SiPh) platform. In order to increase RSOA gain, often the injection current to the RSOA is increased; however, the ultimate gain is limited by gain saturation at high currents. Gain saturation of the RSOA at high currents is due to limited available carrier concentration and over-heating. The gain saturation can cause wall plug efficiency of the laser device to drop.
An object of the present invention is to overcome the shortcomings of the prior art by providing a high power external cavity tunable laser with multiple RSOA's and multiple outputs.